The steam generators of pressurized-water nuclear reactors have a bundle of U-shaped tubes, the ends of which are crimped inside holes passing through a tube plate, and are flush with one of the sides of the tube plate forming its inlet side. A water box is delimited, in the bottom part of the steam generator, by the tube plate, the inlet side of which forms the top wall of the water box.
The pressurized water constituting the reactor coolant flows inside the tubes of the bundle, which tubes ensure thermal contact, in the part of the steam generator located above the tube plate, between the pressurized cooling water and the feed water in contact with the external surface of the tubes. The heat of the reactor cooling water thus ensures that the feed water is heated and vaporized.
The tubes of the bundle form a part of the barrier separating the primary cooling water of the reactor from the feed water. It is therefore necessary to check the walls of the tubes and ensure that they are undamaged, so as to prevent any leaks of the pressurized water in which radioactive elements are suspended, inside the secondary part of the steam generator containing the feed water.
The steam generators of pressurized-water nuclear reactors are intended to operate for very long periods during which their tubes may be subject to a certain degrading action, for example as a result of corrosion by the primary water or the feed water.
Cracks affecting the wall of the tubes may appear in certain zones and in particular in the vicinity of the outlet side of the tube plate. Such cracks result in frequently troublesome leaks of primary water into the feed water.
When such leaks are detected, the tubes of the steam generators affected by leaks are repaired during a down period reserved for maintenance of the reactor. These repairs may be performed by lining the cracked area of the tube. Such an operation consists in fixing a sleeve inside the tube, in the area which is cracked. The tube is expanded at each of its ends, on either side of the cracked area.
To ensure that there is a perfectly leak-proof joint between the sleeve and the tube, a circular weld is effected in the vicinity of each of the ends of the sleeve, so that there is a metallurgical type joint between the sleeve and the tube. This weld is preferably effected by means of the TIG method, i.e., a method using a tungsten electrode and an inert-gas atmosphere in the welding zone.
The quality and sealing action of the joint depend on the quality of the weld effected, so much so that it is preferable to check carefully the condition of the welds joining the sleeves and the tubes, before again starting up the steam generator.
The inside of the steam generator tubes and the walls of the water box are fairly heavily contaminated after a certain period of operation of the steam generator, such that the fitting and final welding of the sleeves require precautionary measures to prevent the personnel carrying out the repairs from being exposed to large doses of radioactivity.
Tools have been designed to perform the fitting and final welding of the sleeves inside contaminated steam generator tubes.
On the other hand, a testing device for checking the condition of the welds performed on the sleeves and preventing contamination of the personnel used to carry out these checks is not known.
Handling devices are known which can be positioned inside the water box of a steam generator and which allow a tool or a test probe to be introduced inside any tube of the bundle, via its end flush with the inlet side of the tube plate.
However, no device is known for positioning and using a non-destructive weld testing probe such as an ultrasonic probe, in order to scan a circular zone of limited length inside a tube.